Gas insulated High Voltage bushings are used for carrying current at high potential through a plane, often referred to as a grounded plane, where the plane is at a different potential than the current path. Bushings are designed to electrically insulate a high voltage conductor, located inside the bushing, from the grounded plane. The grounded plane can for example be a transformer tank or a wall, such as for example a High Voltage Direct Current (HVDC) valve hall wall. An example of a gas isolated bushing is the GGFL, air to air bushing, by ABB.
In a gas filled bushing with a free hanging conductor, for example, a wall bushing, the maximum deflection of the conductor in the longitudinal center of the bushing influences the inner diameter of the bushing which affects the outer diameter of the bushing. In order to prevent flashovers, the higher the maximum deflection is the larger the inside diameter of the bushing has to be. Inside of the bushing, different field control shields are arranged to handle the electrical fields. The field control shields will not work as designed if the conductor is not in the radial center or close to the radial center of the bushing. There is thus a need to minimize the deflection of the conductor in very long bushings.
The static deflection of the conductor is generated by gravity and mass of the conductor itself. The conductor in the bushing is in the form of a tube fixed in both ends. The deflection of a horizontally or near horizontally placed tube is dependent on material constants of the conductor tube (Young's modulus and density), length, wall thickness and diameter of the tube.
The conductor is dimensioned to conduct a current i.e. for a given current and resistivity, the cross sectional area of the conductor is given. For a conductor of a given outer diameter, the wall thickness will be determined by the cross sectional area of the tube. The length is set by the length of the bushing which is determined by external electric requirements e.g. voltages and flashover distances. For large currents it is in principle only possible to use copper or aluminium or alloys thereof in the conductor. This will determine the material parameter which will then set the maximum stiffness of the material. Almost all material parameters and construction parameters are set by the electric requirements of the bushing.
To minimize the static deflection of the conductor at the longitudinal center, a number of solutions have been proposed. The tension of the conductor can be increased but this has only a limited effect on the static deflection. Horizontally moving the fixation point, where the conductor is fixed onto the end flange of the bushing, up from the radial center of the bushing will reduce the deflection at the longitudinal center point. The increasing voltages and very high power distributions that today's equipment has to handle make today's bushing very long, 10-20 m or even longer. For very long bushings, with large static deflection, the required shift of fixation point to solve the static deflection problem becomes too large to be practical.